We've used a couple of the little Panasonic switches in the past. I know they do doors but can't be certain if they will handle voip directly.

Siemens dect might be an option too, I seem to remember there is a dect compatible door unit about. We've got the siemens gigaset in the office and it happily handles one pstn, several voip and 4 extensions.

Its tooo old to understand VOIP but does have the door phone feature that you require. You'll need a system phone in order to program it up & it comes as a basic 1: 6 system (1 exchange lines. 6 extensions) but can be expanded upto 4: 12.

A system can be had on a well known auction site for around the £70 mark and there are still loads of spare PCBs/PSUs/Option cards around ..... I might even be able to help you with a full set of manuals......